Ambassador and politician showing fibre (4)
I believe the answer is:
hemp
'fibre' is the definition.
(hemp is a kind of fibre)
'ambassador and politician' is the wordplay.
'ambassador' becomes 'he' (His or Her Excellency).
'and' says to put letters next to each other.
'politician' becomes 'mp'.
'he'+'mp'='HEMP'
'showing' is the link.
(Other definitions for hemp that I've seen before include "Marijuana" , "Narcotic drug" , "Rope-making fibre" , "Plant for rope or pot" , "Rope, cannabis, plant" .)